Catalytic systems are employed in automotive vehicles to convert exhaust gas components like carbon monoxide, nitrogen oxides and hydrocarbons into other more environmentally desirable gases. Generally, such systems employ three-way catalysts (TWC) to catalyze such conversions and they operate efficiently at temperatures greater than about 350.degree. C. As these TWC age, however, their performance deteriorates. Unacceptably reduced performance of the catalyst is usually detected when a vehicle fails to pass an emission test which is conducted periodically. It is now sometimes required by legislation to include a monitoring device into the automobile system to assess the performance of the catalyst on a continuous basis during operation of the vehicle. Such "on-board" monitoring would allow the catalyst to be replaced when its performance was shown to be less than acceptable. A device which would operate in such a monitoring system must be able to reproducibly and accurately function for an extended period of time in the harsh environment of automotive exhaust gas systems, in particular within the catalytic converter. Further, this desired device should be capable of being employed within a cost-efficient monitoring system capable of assessing the performance of an individual catalyst brick while imposing a minimum restriction on the design of the overall emissions control system. The present invention catalyst monitoring system meets these objectives.